By creating a service agreement you'll be able to outline what service and goods your customer will be receiving. You can also then enlist what penalties the customer will inquire if the cancel or break the agreement. This will help bind the secured services and products offered to your customers.
It is not always necessary to have one but if you are offering free products and / or extended services, you will want to recover the initial liability cost for the products or labor offered if the customer fails with the agreement, then, this will be a way to protect your assets. Your service agreement is totally up to you, with your creativity, and what you want to offer to your customers. This is usually a good thing to go by to acquire customer retention.